Virtual photons

The electron and nucleon interact by the electromagnetic
force, the carrier of this is the virtual photon as has different properties to ordinary
photons. Take for example two electrons. These repel each other due to the
electromagnetic force, we say that there is a mediator or exchange
particle which is transferred between them, the photon. If one imagines two ice
skaters facing each other and one throws a ball to the other person both skaters will move
apart, just as two electrons would repel each other.

When delving inside the proton (or neutron) it is not the
electron which actually 'probes' the nucleon but the photon. An electron gives some
of its energy (and so loses some of its momentum) to the photon. The more momentum
which is transferred to the photon, the more energy it has and so the shorter the
wavelength of the photon. One can imagine that a longer wavelength photon will only 'see'
the whole nucleon and so be elastically scattered, but for shorter wavelength photons it
can 'see' the constituents of the nucleon, the quarks inside. This is why physicists
want to build larger and larger accelerators, so that they can see more and more of the
structure of particles.